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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2019-05-09T02:29:54.1566560+00:00 -->
<!-- Version            : 3.4.7044.22220 -->
<!-- Package ID         : ee72f2a51b5a480988fafc3cd3341ac4 -->
<!-- Copyright (c) 2019 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="arpo_GrantIncome" name="GrantIncome" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arpo_CumulativeEffectOfChangeInAccountingPrincipleInPeriodOfAdoption" name="CumulativeEffectOfChangeInAccountingPrincipleInPeriodOfAdoption" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arpo_IncreaseDecreaseInPrepaidResearchAndDevelopmentContracts" name="IncreaseDecreaseInPrepaidResearchAndDevelopmentContracts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arpo_CommonStockTextBlock" name="CommonStockTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
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  <xsd:element id="arpo_LicenseAgreementTextBlock" name="LicenseAgreementTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arpo_GrantIncomePolicyTextBlock" name="GrantIncomePolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arpo_ZetaAcquisitionCorpIIMember" name="ZetaAcquisitionCorpIIMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arpo_CapitalStockConversionBasisRatio" name="CapitalStockConversionBasisRatio" type="xbrli:pureItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
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  <xsd:element id="arpo_SummaryOfSignificantAccountingPoliciesLineItems" name="SummaryOfSignificantAccountingPoliciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arpo_SummaryOfSignificantAccountingPoliciesTable" name="SummaryOfSignificantAccountingPoliciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arpo_AccountingStandardsUpdate201807Member" name="AccountingStandardsUpdate201807Member" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arpo_AccruedProjectCostsCurrent" name="AccruedProjectCostsCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arpo_ClassOfWarrantOrRightTerm" name="ClassOfWarrantOrRightTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arpo_ClassOfWarrantOrRightExpirationDate" name="ClassOfWarrantOrRightExpirationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arpo_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorizedAnnualPercentage" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorizedAnnualPercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arpo_TwoThousandSeventeenStockOptionAndIncentivePlanMember" name="TwoThousandSeventeenStockOptionAndIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
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  <xsd:element id="arpo_CashPaidForAmountsIncludedInMeasurementOfOperatingLeaseLiabilitiesAbstract" name="CashPaidForAmountsIncludedInMeasurementOfOperatingLeaseLiabilitiesAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arpo_NumberOfSharesReservedForIssuanceAnnualIncreasedPercentage" name="NumberOfSharesReservedForIssuanceAnnualIncreasedPercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
</xsd:schema>
